1. Field of the invention
This invention relates to an optical pickup device which is useful in an information recording and/or reproducing apparatus such as a compact disc reproducing apparatus, a video disc reproducing apparatus and the like.
2. Description of the prior art
FIG. 9 shows a conventional optical pickup device. The device of FIG. 9 comprises a semiconductor laser device 1, a first diffraction device 2, a second diffraction device 23, a collimating lens 4, an object lens 5, and a photodetector 17. A light beam from the semiconductor laser device 1 is diffracted by the first diffraction device 2 to produce three separate light beams, one of which is the zero-order diffracted beam (hereinafter, referred to as "the main beam"), and the others of which are the first-order diffracted beams (hereinafter, referred to as "the sub-beams") in the positive and negative directions which are substantially orthogonal to the sheet of the drawing of FIG. 9. These three separate beams are further diffracted by the second diffraction device 23. The resulting zero-order diffracted beam of each of the above-mentioned separate beams enters the object lens 5 through the collimating lens 4, and is focused on a recording medium 6 The main beam is focused on a pit of the recording medium 6. The two sub-beams, which are positioned symmetrically with respect to the above-mentioned main beam, are focused on the recording medium 6 in such a manner that they shift to a large extent in the tracking direction of the recording medium 6 and to a small extent in the radial direction. The beams reflected from the recording medium 6 pass through the object lens 5 and the collimating lens 4, and are diffracted by the second diffraction device 23. The resulting first-order diffracted beams are introduced into the photodetector 17.
As shown in FIG. 10A, the diffraction device 23 is divided into two diffraction regions 23a and 23b by a division line 23c, when viewed from the side of the recording medium 6. The regions 23a and 23b have a number of grating lines which are inclined with respect to the division line 23c and which are symmetrical about the division line 23c. The photodetector 17 is divided into six regions 17a to 17f, as shown in FIG. 10B. The division line 23c elongates in the radial direction of the recording medium 6.
When a beam from the semiconductor laser device 1 is precisely focused on the recording medium 6 or set at the correct focus, the resulting main beam which has been diffracted by the region 23a of the diffraction device 23 is focused on the division line A.sub.1 of the photodetector 17 to form a spot Q.sub.1, and the resulting main beam which has been diffracted by the region 23b of the diffraction device 23 is focused on the division line B.sub.1 to form a spot Q.sub.2. The resulting sub-beams are focused on the regions 17e and 17f of the photodetector 17. When output signals of the photodetecting regions 17a to 17f are represented respectively as S.sub.1a to S.sub.1f, a focus error signal is obtained by calculating (S.sub.1a +S.sub.1d)-(S.sub.1b +S.sub.1c), a tracking error signal is obtained by calculating (S.sub.1e -S.sub.1f), and a pit signal (i.e., an information signal) is obtained by calculating (S.sub.1a +S.sub.1b +S.sub.1c +s.sub.1d).
FIG. 11 shows another conventional optical pickup device. The optical pickup device of FIG. 11 is different from the above-mentioned conventional device in the configurations of a second diffraction device 33 and a photodetector 27 The configuration of the grating lines of the diffraction device 33 and that of the photodetector 27, which are viewed from the recording medium 6, are shown in FIGS. 12A and 12B, respectively The diffraction device 33 is divided into two regions 33a and 33b by a division line 33c which elongates in the radial direction. The regions 33a and 33b have a number of grating lines which are at right angles to the division line 33c, and the grating period of the region 33a is different from that of the other region 33b. The photodetector 27 is divided into six regions 27a to 27f. When a beam from the semiconductor laser device 1 is precisely focused on the recording medium 6 or set at the correct focus, the resulting main beam which has been diffracted by the region 33a is focused on the division line A.sub.2 to form a spot R.sub.1 thereon, and the resulting main beam which has been diffracted by the region 33b is focused on the division line B.sub.2 to form a spot R.sub.2 thereon. The resulting sub-beams are focused on the photodetecting regions 27e and 27f. When output signals of the photodetecting regions 27a to 27f are represented respectively as S.sub.2a to S.sub.2f, a focus error signal is obtained by calculating (S.sub.2a +S.sub.2d)-(S.sub.2b +S.sub.2c), a tracking error signal is obtained by calculating (S.sub.2e -S.sub.2f), and a pit signal is obtained by calculating (S.sub.2a +S.sub.2b +S.sub.2c +S.sub.2d).
In the conventional optical pickup devices with the above-mentioned structures, the spots Q.sub.1 and Q.sub.2 (R.sub.1 and R.sub.2) based on the beams reflected from the recording medium 6 must be very precisely formed on the division lines of the photodetector 17(27). To achieve this, a delicate adjustment must be carried out so that the diffraction device 23(33) and the photodetector 17(27) respectively can be disposed at a given position. However, in order that the diffraction device 23(33) and the photodetector 17(27) are constructed to be moved separately or independently from the diffraction device 23(33), there must be a supporting structure by which the photodetector 17(27) can be freely moved. This makes the entire structure of the pickup device complicated, causing difficulties in obtaining a light-weight, miniaturized pickup device. Moreover, a number of positioning parts are needed, which makes the production process of the pickup device complicated and makes the production cost expensive.
To solve these problems, the inventors of this invention designed to incorporate both the semiconductor laser device 1 and the photodetector 17(27) into the same package so that the positioning of the spots Q.sub.1 and Q.sub.2 (R.sub.1 and R.sub.2) on the division lines of the photodetector 17(27) can be carried out by the positional adjustment of the diffraction device 23(33) alone. However, in a optical pickup device with such a structure, the slight shifting of the positions of the photodetector 17(27) from those of the initial plan makes it impossible to form the beam spots at the correct positions of the photodetector 17(27), resulting in a focus offset. To remove this focus offset, the position of the diffraction device 23(33) must be moved linearly and/or rotationally with respect to other components such as the semiconductor laser device 1 to shift the spots on the photodetector 17(27), so that the focus error signal becomes zero when the beam from the semiconductor laser device 1 is at the correct focus on the recording medium 6. However, the two spots on the photodetector 17(27) which are formed based on the main beams shift at the same time, resulting in that the position of each beam spot cannot be independently adjusted without the simultaneous shifting of these beam spots on the photodetector 17(27). Moreover, there is a possibility that the shifting of the two spots are countervailed on the focus error signals corresponding thereto. To avoid this, the diffraction device 23(33) must be moved to a great extent in the Y-direction (FIGS. 10B and 12B). Especially, in the optical pickup device shown in FIG. 11, the length of each of the divided regions of the photodetector 27 in the y-direction is short. Therefore, when a great focus offset occurs and the diffraction device 33 is moved to a great extent in the y-direction to compensate the said focus offset, the beam spots R.sub.1 and R.sub.2 on the photodetector 27 shift to a great extent in the y-direction and slip out of the photodetecting regions on which these spots must be formed.
Moreover, because the diffraction device 23 (33) must be moved linearly to compensate for the focus offset phenomenon, the photodetector 17(27) is required to have a large enough size to receive the beam spots thereon, which makes the production cost thereof expensive.
FIG. 13 shows an optical pickup device which is disclosed in U.S. Ser. No. 07/282,109, European Patent Appln. No. 88311665.9 and Canadian Patent Appln No. 585,356. In the pickup device of FIG. 13, the diffraction device 43 consists of two diffraction regions 43a and 43b which are formed by dividing the whole area of the diffraction device by a division line 43c. The diffraction regions 43a and 43b have a number of grating lines which elongate in the track direction of the recording medium 6 (X--X' direction in FIG. 13). The division line 43c elongates in the direction (Y--Y' direction in FIG. 13) which is perpendicular to the track direction. The photodetector 37 is divided into five regions 37a to 37e. The division line 37f extends in the Y--Y' direction to separate the regions 37a and 37b.
The main beam which has been diffracted by the diffraction region 43a is focused on the division line 37f to form a spot S.sub.1, and the main beam which has been diffracted by the diffraction region 43b is focused on the region 37c to form a spot S.sub.2. The sub-beams are focused on the regions 37d and 37e to form spots S.sub.3 to S.sub.6. When a beam from the semiconductor laser device 1 is precisely focused on the recording medium 6, the spots S.sub.1 to S.sub.6 are formed as tiny spots as shown in FIG. 14B. In contrast, when the distance between the recording medium 6 and the object lens 5 becomes small (or large), the spots S.sub.3 to S.sub.6 are formed so as to be extended in semicircular shapes as shown in FIG. 14B (or in FIG. 14C). The regions 37a to 37e on
which spots S.sub.3 to S.sub.6 are formed produce signals S.sub.a to S.sub.e, respectively. A focus error signal is obtained by calculating (S.sub.a -S.sub.b), a tracking error signal by calculating (S.sub.d -S.sub.e), and an information signal by calculating (S.sub.a +S.sub.b +S.sub.c).
The optical pickup device of FIG. 13 has a drawback in that a spurious signal is generated in the focus signal, tracking error signal and information signal. This will be described with reference to FIGS. 15 and 16. A light beam emitted from the semiconductor laser device 1 is divided into a main beam and two sub-beams by the diffraction device 2, and enters in the diffraction device 43. For example, the resulting first-order diffracted beam diffracted by dividing the diffraction region 43b propagates as indicated by the phantom lines A, as if it is a light beam emitted from the spot S.sub.2 toward the diffraction device 43. Therefore, this first-order diffracted beam is focused by the lenses 4 and 5 on the position of the recording medium 6, a position which corresponds to the spot S.sub.2 (i.e., the position is the image point of the spot S.sub.2). As shown by the phantom lines B, this first-order diffracted beam is reflected by the recording medium 6 to enter into the diffraction device 43 through the lenses 5 and 4, and the resulting zero-order diffracted beam is focused on the photodetector 37 as the spot S.sub.2. Similarly, the resulting first-order diffracted beam diffracted by dividing the diffraction region 43a is reflected by the recording medium 6, and then focused on the photodetector 37 as the spot S.sub.1.
The first-order diffracted beams which are diffracted by the diffraction device 43 are not the light beams to be used for detecting signals. When these first-order diffracted beams are once received by the photodetector 37, therefore, a spurious signal is generated in the focus error signal and information signal. This causes the focus control to be incorrectly conducted, and an incorrect information signal to be produced. Also, the diffraction device 43 produces first-order diffracted beams based on the sub-beams which have been diffracted by the diffraction device 2, so that a spurious signal is also generated in the tracking error signal, thereby impeding the tracking control In this way, the first-order diffracted beams produced by the diffraction device 43 cause a spurious signal in detection signals so that the optical pickup device cannot function properly.